A mobile communication system has been evolved to a wireless packet data communication system for providing a data service and a multimedia service as well as a voice service. So, mobile communication systems which provide a high speed packet data service such as a high speed downlink packet access (HSDPA) system and a long term evolution (LTE) system, and the like which are proposed in a 3rd generation partnership project (3GPP) have been developed.
In a mobile communication system, a mobile station (MS) may need to acquire synchronization with a base station (BS) in order to perform a communication with the BS, and a process of acquiring synchronization between the MS and the BS will be described below.
A frame structure in a downlink frame in a conventional LTE system supporting a frequency division duplexing (FDD) scheme will be described with reference to FIG. 1.
FIG. 1 is an example of a frame structure in a downlink frame in a conventional LTE system supporting an FDD scheme.
Referring to FIG. 1, a frame structure in FIG. 1 is a frame structure in which a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) for acquiring synchronization in a downlink of a conventional LTE system supporting an FDD scheme.
In FIG. 1, a frame length of a frame 101 can be 10 ms, and the frame 101 can include 10 sub-frames. A sub-frame length of a sub-frame 103 can be 1 ms, and the sub-frame 103 can include 2 slots. A slot length of a slot 105 can be 0.5 ms. In a frame structure supporting an FDD scheme, a PSS 111 can be transmitted on the last orthogonal frequency division multiplexing (OFDM) symbol included in each of a sub-frame #0 and a sub-frame #5, and an SSS 113 can be transmitted on an OFDM symbol just before an OFDM symbol on which the PSS 111 is transmitted. An MS can acquire frame synchronization by receiving the PSS 111 and the SSS 113.
A frame structure in a downlink frame in a conventional LTE system supporting an FDD scheme has been described with reference to FIG. 1, and a slot structure in a downlink frame in a conventional LTE system supporting an FDD scheme will be described with reference to FIG. 2.
FIG. 2 is an example of a slot structure in a downlink frame in a conventional LTE system supporting an FDD scheme.
Referring to FIG. 2, (A) illustrates a slot structure which can use a normal cyclic prefix (CP) on an OFDM symbol, and 7 OFDM symbols can be1 transmitted through one slot. In FIG. 2, (B) illustrates a slot structure which can use an extended CP on an OFDM symbol, and 6 OFDM symbols can be transmitted through one slot. That is, in an LTE system, the number of symbols included in a slot can be changed according to a type of a CP.
In the LTE system, a slot length of one slot can be 0.5 ms, and slot length may not be changed according to a type of a used CP.
Table 1 expresses variables necessary for calculating a CP length and a symbol length and values thereof
TABLE 1variablelengthdescriptionTS0.00325 usLTE basic time unitTU2048 TS ≈ 66.7 usLTE symbol lengthTCP2048 TS ≈ 66.7 usCP length of the first normal CP144 TS ≈ 4.7 usCP length of each of remaining 6 normalCPsTCP-e 512 TS ≈ 16.7 usExtended CP length
A slot structure in a downlink frame in a conventional LTE system supporting an FDD scheme has been described with reference to FIG. 2, and a frame structure in a downlink frame in a conventional LTE system supporting a time division duplexing (TDD) scheme will be described with reference to FIG. 3.
FIG. 3 is an example of a frame structure in a downlink frame in a conventional LTE system supporting a TDD scheme.
Referring to FIG. 3, a frame length of a frame 301 can be 10 ms, and can include 2 half-frames of which a half-frame length can be 5 ms. A half-frame can include 5 sub-frames. Here, a sub-frame length of a sub-frame can be 1 ms.
A frame structure in a downlink frame in a conventional LTE system supporting a TDD scheme has been described with reference to FIG. 3, and a difference between a frame structure in a downlink in a conventional LTE system supporting a TDD scheme and a frame structure in a downlink in a conventional LTE system supporting an FDD scheme will be described with reference to FIG. 4.
FIG. 4 is an example of a special sub-frame structure in a downlink frame in a conventional LTE system supporting a TDD scheme.
Referring to FIG. 4, unlike a frame structure supporting an FDD scheme, a frame structure supporting the TDD scheme can include a special sub-frame 401 for acquiring synchronization, such as for adjusting a timing, and the special sub-frame 401 can be included in each of the first sub-frame and the sixth sub-frame.
As described above, in an LTE system which uses an OFDM transmission scheme, an MS can receive a special sub-frame in a frame structure of FIG. 1 or FIG. 3 to acquire synchronization.
Meanwhile, a portable terminal which uses a high speed packet service such as a smart phone, and the like has been widely disseminated, so data traffic can exponentially increase in a network, and it can be expected that the data traffic will much more exponentially increase due to more application services which use the portable terminal.
So, in a mobile communication system which can provide a high speed service such as the LTE system, a BS which supports a small cell which has a relatively small service coverage such as a pico cell, a femto cell, a wireless local access network (WLAN), and the like can be additionally deployed for increasing total capability and distributing data traffic, and technologies for distributing data traffic of a BS in a conventional macro cell into a BS in a small cell to maximize use of the BS in the small cell can be used.
Like this, a network structure in which a BS of a macro cell can be co-located with a BS of a small cell in a mobile communication system called a distributed cell structure.
In the distributed cell structure, handover can frequently occur due to a movement of an MS and a service area of a cell can be dynamically changed, so a timing error between BSs of cells can be more likely to occur. The timing error between the BSs can result in a service interruption or a network re-connection of the MS.
So, there can be a need for decreasing a timing error between BSs in a network with a distributed cell structure.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.